Optical system for reflecting



.nu-M SR C SEARCH ROOM Allg. 7, 1951 p H TAYLOR 2,563,433

OPTICAL SYSTEM Foa REFLECTING MIRROR TELEscoPE 7 7 c 2 Filed April 7. 195o Patented Aug. 7, 1951 UNITED STATES PATENT OFFICE OPTICAL SYSTEM FOR REFLECTING MIRROR TELESCOPE 3 Claims.

This invention relates to the use of lenses of zero-power for the correction of mirror aberrations in a mirror telescopic system.

It is an object of this invention to provide a suitable lens of zero-power as a corrector plate for the correction of mirror aberrations by a transmitting element.

The correction of mirror aberrationsby a transmitting element was first accomplished by Schmidt who used an aspheric plateV at an aperture stop located at the center of curvature of a concave spherical mirror. D. D. Maksutov and A. Bouwers, some years later, developed meniscus correctors having all-spherical surfaces. The suggestion has also been made of combining menisci with Schmidt type plates to achieve an even higher order of correction.

Figure 1 is a diagram showing a typical system of the Maksutov type. This particular example is shown monocentric for simplicity, though more generalized forms are common.

Figure 2 is a graph showing the spherical aberrations of a zero power lens formed from a single piece of glass.

Figure 3 is a diagram of a 44.5 inch f/ 6.4 tele scope embodying the present invention.

Figure 4 is a graph showing color curves for the system of Figure 3.

In Figure 1, a spherical mirror l is positioned behind a meniscus corrector 2, with an aperture stop 3 in front of the meniscus corrector 2. All surfaces and the image eld I are centered at C in the axis of the system in the aperture stop plane.

. When an attempt is made to design an instrument of the Maksutov type, of Figure 1, it is seen at once that an aberration-free scheme can be achieved easily at, say, about f/lO and with modest apertures of four to six inches. If the designer then attempts to increase the speed and/or the focal length, three. difficulties are encountered:

First, color becomes unpleasantly large. This can be controlled by making the meniscus a cemented doublet comprised of two glasses having the same index for sodium light, but with different dispersions. This device has only a limited application for, at very high aperture ratios (f/3 or faster), the lateral color introduced at the cemented surface, whose center of curvature is far removed from that common to the others, becomes too large for high-quality imagery.

Secondly, one of the parameters for the control of spherical aberration (the only one in the monocentric case) is the thickness of the meniscus. In large, fast systems, the unpleasant alternatives of a relatively thin lens with steep (full aperture) 2 This increases steadily with aperture and speed,

until at a focal length of two inches, working at f/l, the zonal is about times that permlssible for perfect image production.

A new approach to the problem according to the present invention is to employ, instead of the negative power menisci, a lens of zero power which contributes solely to the spherical aberration produced by the mirror or mirrors of the system.

The present invention may be more fully understood by reference to Figures 2, 3 and 4.

A few simple trials show that a zero-power lens cannot be made of a single piece of glass, for such a lens is always afflicted with positive spherical aberration :lust like the mirror. The combined system will then be worse than the mirror alone unless the "corrector is allowed to degenerate into a plane parallel plate. The result of bendings is shown qualitatively by the curve 5 of Figure 2.

Negative spherical aberration can, however, be achieved with a compound zero power lens. An exceedingly simple but useful form, and the one entering into the present invention is that of a symmetrical cemented triplet made of a central equi-concave flint element, flanked by two identical crown elements. The inverse of the achromatization trick mentioned above is used, and two glasses having the same V values but whose indices differ widely are chosen. Throughout the corrective process. symmetry is retained, not only because it is the natural form for zeropower (or any other lens used at unit magnication) but also because coma, lateral color, and distortion are, thereby, automatically reduced to zero.

One preferred system embodying the present invention is one of eight inches aperture and 44.5 inches focal length, assembled as shown in Figure 3.

Here, an apertured primary concave mirror I0 is used, reflecting light to a smaller secondary concave mirror ll and then through the aperture in the primary mirror to an image field I 3 behind the primary mirror I0. Corrections are provided by a compound zero power lens I2 formed from a central equi-concave flint glass element I4 flanked by two identical crown glass Color curves of this system are shown in Figure 4 where curve P is the paraxial color curve, and curve M is the marginal color curve. Between 4000 and 6600 angstroms the paraxial curve P elements I5 and I6, the interfaces of the latter 5 G Xhbts the general type 0f achromatism assobeing cemented to the central element. In the @lated Wlti HIJOChIOmatlC Systems- IS erratic figure, the various lens and mirror surface radii Course neeq not be Cause for any oncem as thg r are each numbered by subscripts from front to Scatterfng 15 brought abfmt by rounding'o rear of the system; d1, d2 and d3 are respectively gfmfs m the mdies- Dlmlg design a Cheqk can the axial thicknesses of the front crown element 10 tl'enaf arneal axslge ypgsfilydiin t entral ilin element I4 n the rear f rowrlileelement I6 rtespectively, wlrrcililiej d4 is the rectonstfor .e tight", This procedure gave rise axial distance between the primary mirror sur- Eg16653?rgtglnhrneg'roygel face and the surface r4, and d5 is the axial distance between secondary mirror surface r6 and l5 lsfxarn;egglrfrggggergamsm overthe image eld I. e

. The curves show, however, the great importthBeause bof 111; cn'l cylnder bockd mit? ance of obtaining indices for each optical disc, the anz. y ts@ 0 confvex dmlfrtof n accurate to at least unity in the nftn piace. Five 8.8 eclve aper ure o 1s pre erre sys en? 1s 20 units in the sixth place leads to a better instru- 7.0 inches and the speed is f/6.4. The following ment table gives the prescription, and the aberrations Recent interferometric tests of rst quality with their tolerances of the preferred system "Bausch and Lomb Optical glass show that the illustrated 1 n Figure 3. index variation within an optical disc one inch For definition of symbols and abbreviations of 5 thick is about three to five to ilve units in the Table I, the standard text book of E. Conrady sixth place, per inch of surface. Therefore, there entitled Applied Optics and Optical Design. is iittie use insisting on knowing the average in- Oxford University Press, 1929 is referred to. dex to a higher accuracy than this.

Table Prescription V ND Nc N1 Nh NA' r=-22. 236 (cemented) d.: 1.00 DBC-1 58.8 1. e110 1. 60793 1.01832 1. 62837 1.00439 r3: 22. 236 (cemented) di= 3s. 50

= 40.000 Mirror -27. 500 Mirror Mirror 1 (concave pr1m0ry,f= 40 0 The dashed curve Px of Figure 4 is simply that gigigfegnseisncnes. which would be true physically if it could be Diani 6nra`1h01e=s0inehss (clear). ascertained that the tabular F, D and C indices Milnseondary r= 27 5) 45 were really followed by two zeros. The dl'erence A Clear diameterj-B inches (.05 to .'10 holding margin). between this and the marginal plot ShOWn iS gggrfslzggfmhchromatic difference of spherical aberration,

opnclaz constants and aberrations: and is real, being quite independent of the tabular fUlglgolessm U,M=.089941 M indices and dependent only upon the radii and Rear surficeoof mirror l to focal pla1ie=2.9 inches. 50 the accuracy of the traces (eight p1ace) Ful] Hel 1 The advantages of the instrument of the presljv=16v9314 ljv=16-96911 H'='=3S82 ent invention over the `prior telescopes mentioned 'iglg h l'gem are the following:

'Dqggs 55 1. Over a single-mirror Schmidt:

p 1 o PerCent Tolerance Attained (a) Tube 1 ength' f Instead of 2f LA'/= .00030i.0105s 2.6 (b) Spherical corrector.

Dtgilgfgltm? 3. Over a Maksutov meniscus-corrected reflecwhen, kn g5 (a) No zonal spherical aberration.

catalogs ol optiml glass; plate.

DBC-1 is a dense barium crown glass; (c) No secondary color.

v-Nlvfc: (d) Capable of greater speed at a given focal Nn is the index of retraction for the D line o! sodium (5593 J; 70 lefgl'h Whlle holding the aberrations Ne is the index of retraction for the C line of vhydrogen. (6563 Wllhln bounds.

Np is the index of refraction for the Flinc of hydrogen (4861.); (e) Longitudinal placement and tilting of N 1. is the index of refraction for thel h line of mercitiry (4047( the corrector plate relatively uncriticah r i; tllise railuls (lentlrieficarxffellement surface; and 4' over a' cassegrainia'n reflector' d is the axial thickness of an clement. 75 (a) All Spherical Optics.

`SEARCH ROOM (b) Much larger eld. 5. Over refractors having moderately wide elds: (a) No secondary color if used in the F to C region.

6 ment having the same V values but having different indices of refraction.

2. A two mirror telescope comprising from rear to front, an apertured concave primary mirror,

(b) Simplicity of design and manufacture 5 a secondary coaxial convex mirror of smaller (c) No zonal spherical aberration. diameter positioned to reflect light through the (d) Ability to cover several thousand angaperture in said primary mirror to an image field Stroms spectral range. behind said primary mirror, and a compound (e) N0 extra-1111131 aberrations, zero power corrector lens in front of said second- 10 ary mirror and transmitting light to said primary rtrhe dlsfadvlitalgess Stvth rrlsntd tloyr? mirror past said secondary mirror, said zero yes topb ca yberms power lens consisting of a central equi-concave re erre o y num flint glass element flanked 4by two identical crown 1. Alignment of two mirrors. glass elements having convex surfaces cemented 2. (None). to the concave surfaces of said central element, 3. (None). said telescope having the following prescription:

Prescription V Nn NC Nr Nh NA.

r1= 136. 59 d,= 1.00 c-1 58.6 1.5230 1.52036 1.52929 1. 53819 1.51729 r2= 22. 235 (cemented) 1.00 DBC-1 58,8 1.6110 1.60793 1.61832 1. 62837 1. 60439 22.236 (cemented) 1.00 C-1 58.6 1.5230 1.52036 1.52929 1.55819 1.51729 136.59 38.50 -40.000 Mirror d'5- 12.500 r1= -27. 500 Mirror 4. Longer tube length for a given focal length. 5. (a) Spacing of the mirrors must be held clos'ely. (b) Longer tube length than is obtainable in telephotos of identical focal length. (c) Spherical field. (d) Clear aperture about 12% greater than effective aperture.

It should be pointed out that, for some applications, 5 (c) and (d) are no disadvantage and, unless the telephoto type is being discussed, the tube lengths are exactly equal, in both cases, to the focal length.

The telescope with two spherical mirrors and the spherical'triplet corrector element of zero power as described and claimed herein is believed to be superior to any telescope previously designed when the highest quality of images over a field of six degrees or less is desired.

What is claimed is:

1. A two mirror telescope comprising from rear to front, an apertured concave primary mirror, a secondary coaxial convex mirror of smaller diameter positioned to reflect light through the aperture in said primary mirror to an image field behind said primary mirror, and a compound zero power corrector lens infront of said secondary mirror and transmitting light to said primary mirror past said secondary mirror, said zero power lens consisting of a central equi-concave Mirror l (concave primary, r=-40.0):

Clear diameter=8.8 inches Thickness=l.5 inches Diam. central ho1e=3.0 inches (clear) Material= Pyrex Mirror 2 (convex secondary, r=27.5):

lear diamcter=3.6 inches (.05 to .10 holding margin) Thickness=.75 inch Material: Pyrex Where: r is the radius length for an element surface; d is the axial thickness of an element; C'1 is a type of crown glass well known in the art; DBC-l is a type of dense barium crown glass well known in the art;

Nn-x Nr-Ne e Nn is the index of refraction for the D line of sodium (5893 A); a Nc is the index of refraction for the C line of hydrogen (6563 A); Np is the index of refraction for the F line of hydrogen (48061 N 1 is the index of refraction for the ..line of mercury (4047 A.); and

N .y is the index of refraction for the A' line of potassium (7665 3. A two mirror telescope comprising from rear to front, an apertured concave primary mirror, a secondary coaxial convex mirror of smaller diameter positioned to reflect light through the aperture in said primary mirror to an image field behind said primary mirror, and a compound zero power corrector lens in front of said secondary mirror and transmitting light to said primary mirror past said secondary mirror, said zero power lens consisting of a, central equi-concave flint glass element flanked by two identical crown glass elements having convex surfaces cemented to the concave surfaces of said central element, said telescope having the following prescription:

Prescription V N D Nc Ny Nh NA' -27500 Mirror flint glass element flanked by two identical crown glass elements having convex surfaces cemented to the concave surfaces of said central element,

the two crown elements and said flint glass ele- Mirror l (concave primary, ra 40.0):

Clear diamcter=8.8 inches Thickness=l.5 inches Diam. central hole==3.0 inches (clear) Material-Pyrex Mirror 2 (oonvex secondary, f= -27.5):

Clear diameter=3-6 inches (.05 to .10 holding margin) Thickness=-75 inch Material: Pyrex f'=44.41 inches REFERENCES CITED The following references are of record in the le of this patent:

UM=5.160, sin U'M=.08994, f/GA |E N Rear surface of mirror 1 to local plane=2.9 inches 5 UNITED STATES PA TS run ne1d=1 Number Name Date Wthrere d I th f I t r 1,446,634 B611 et a1 Feb. 27, 1923 r is e ra ius eng or an e emen sur ace; dis the axial thickness of an element; 1556982 Wledert Oct' 131 1925 C-l is a type of crown glass well known in the art: 1,967,214 Acht July 24 1934 DBC-1 is a type of dense barium crown glass well known in the art; 10 2,141 884 Sonnefeld Dec 27 1938 V-Nlfjc: 2,336,379 Warmisham Dec. 7, 1943 Nn is the index of refraction for the D line of sodium (5893 JD; 2350112 Houghton May 30 1944 NC is the index or refraction for :he C une of hydrogen 6563 am FOREIGN PATENTS Nr is the index of refraction for the F line of hydrogen (486i1 A.); N i. is the index of refraction for the h line of mercury (4047 A.); o 15 N umher Country Date NA' is the index of refraction for the A line of potassium (7665 A.); 721,462 Germany June 10 1942 f is the focal length and UM is the angleAthat a marginal ray makes with the optical axis after 8831937 France ADL 5, 1943 leaving the last element. 61,355 Denmark Sept. 27, 1943 PHILIP H. TAYLOR. 

